Four color separation is a branch of color photography dealing with separating the three primary colors and black by filtration of the colors and recording the individual color on black and white film such that a photosensitive printing plate may be lithographed or photoengraved with the image of each color for producing printing plates to be used in four color printing.
An inverse or negative image of the color filtered is formed on the film of the copy after it is developed.
The next step of the process is to make a positive transparent film which may be tinted or screened to produce dots or half tones. Dots or half tones are necessary in order to photoengrave the plates since printing requires the positioning of dots onto the printed paper and not a solid mass of ink. The positive film allows more accurate evaluation of the tones.
Problems occur in identifying each film in each step of the process where each time the image is inversed or positive becomes negative, and negative becomes positive. Problems occur in aligning the film from step to step as only positive register marks give accurate alignment. Negative register marks are wider due to the physical spreading of light, and it is difficult to accurately register using the wider negative register marks. Accurate registration is necessary to give a true color reproduction.
In addition, viewing a piece of black and white film does not reveal with certainty the color that the film has recorded in it, therefore many mistakes may occur through careless handling of the film and mismarking of the film.
Previous methods of identifying the color recorded on the film involved rotation of a register mark having a black bar in a certain quandrant about the cross-hairs each time a photograph is taken of a new color. This requires physical handling of the register mark which would allow misalignment of the mark so that alignment of the film could not accurately be done in proceeding steps. In addition, rotation of the marks requires a great deal of time and may allow error in the identification of the film.
Other methods of identifying the film include human examination and scratching a mark on the film.
Color bars used for color balance in exposing the film are removed before engraving a plate and do not stay with the film to provide a permanent record.